379 research outputs found
Canada: policymaking, down home no more?
Ending decades of neoliberal policy, the new Liberal government of Canada aims to restore the protection that fishermen have lost to foreign corporate interests
Muscle as a meta-material operating near a critical point
Passive mechanical response of skeletal muscles at fast time scales is
dominated by long range interactions inducing cooperative behavior without
breaking the detailed balance. This leads to such unusual "material properties"
as negative equilibrium stiffness and different behavior in force and
displacement controlled loading conditions. Our fitting of experimental data
suggests that "muscle material" is finely tuned to perform close to a critical
point which explains large fluctuations observed in muscles close to the stall
force.Comment: Accepted for publication in Physical Review Letter
Quantitative imaging of anisotropic material properties with vectorial ptychography
Following the recent establishment of the formalism of vectorial ptychography
[Ferrand et al., Opt. Lett. 40, 5144 (2015)], first measurements are reported
in the optical range, demonstrating the capability of the proposed method to
map the four parameters of the Jones matrix of an anisotropic specimen, and
therefore to quantify a wide range of optical material properties, including
power transmittance, optical path difference, diattenuation, retardance, and
fast-axis orientation.Comment: 5 figures, accepted for publication in Optics Letter
Angular Momentum Evolution of Stars in the Orion Nebula Cluster
(Abridged) We present theoretical models of stellar angular momentum
evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades.
We demonstrate that observations of the Pleiades and Hyades place tight
constraints on the angular momentum loss rate from stellar winds. The observed
periods, masses and ages of ONC stars in the range 0.2--0.5 M, and the
loss properties inferred from the Pleiades and Hyades stars, are then used to
test the initial conditions for stellar evolution models. We use these models
to estimate the distribution of rotational velocities for the ONC stars at the
age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades
distributions of rotation rates are not consistent if only stellar winds are
included. In order to reconcile the observed loss of angu lar momentum between
these two clusters, an extrinsic loss mechanism such as protostar-accretion
disk interaction is required. Our model, which evolves the ONC stars with a
mass dependent saturation threshold normalized such that at 0.5 \m, and which includes a distribution of disk lifetimes
that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This
model for disk-locking lifetimes is also consistent with inferred disk
lifetimes from the percentage of stars with infrared excesses observed in young
clusters. Different models, using a variety of initial period distributions and
different maximum disk lifetimes, are also compared to the Pleiades. For
disk-locking models that use a uniform distribution of disk lifetimes over the
range 0 to , the acceptable range of the maximum lifetime is Myr.Comment: 21 pages, 7 figures, submitted to Ap
Efficient binary reconstruction for non destructive evaluation using gammagraphy
International audienceThe localization and the sizing of 3D flaws within a homogeneous metallic media is a major task for non destructive evaluation (NDE). This paper adresses the problem of the reconstruction of such flaws using an efficient binary algorithm. Basically, the method rests on the fact that a simple binary constraint suffices for an accurate and robust reconstructions in the context of NDE. A heuristic minimization, computationally attractive, is designed in order to provide fast reconstructions. The proposed algorithm is compared with standard binary (the iterated conditional mode algorithm) and non binary (penalized approach with convex potentials Gibbs random fields) reconstruction techniques
Ptychography in anisotropic media
International audiencePtychography is described in the context of polarized light probing anisotropic specimen, i.e., showing properties of birefringence and/or diattenuation. We establish an optimization strategy using a vectorial formalism. A measurement scheme using a set of linearly polarized probes and linear polarization analyzers is proposed, allowing to retrieve the full anisotropy map of the specimen
Imaging of highly inhomogeneous strain field in nanocrystals using x-ray Bragg ptychography: A numerical study
International audienceX-ray ptychography is a lensless microscopy method able to provide extended field of view with spatial resolution above the diffraction limit. A series of intensity coherent diffraction patterns measured in the far field is used to obtain the numerical deconvolution between the sample scattering contrast and the illumination function. The measurements are performed with a finite-size beam spot scanned across the sample. The scan step, smaller than the beam size, ensures a high redundancy in the data set, which allows for the convergence of the iterative inversion algorithm. This work explores the possibility to use ptychography for the investigation of strained crystals by means of coherent x-ray Bragg diffraction, taking advantage of the high sensitivity to the atomic displacement fields. The Bragg diffraction scattering contrast is described by an effective complex-valued electron density, where the phase holds the information on the displacement field. The detailed two-dimensional numerical study of Bragg ptychography is presented, both for the known and unknown illumination cases. It demonstrates the high robustness of the ptychographical iterative engine for highly nonhomogeneous strain fields. In particular, the local information is extracted from the individual diffraction patterns to calculate the modulus and phase estimates of the electron density, which are further used to constrain the newly derived algorithm. From this work, it is foreseen that Bragg ptychography when experimentally feasible, will open the way to the nondestructive imaging of strain fields at the nanoscale
Superfluid Helium Flow in Porous Media
Superfluid helium is primarily used in the field of applied superconductivity. Given the complexity of the magnet geometry and the scales involved, a real 3D simulation of heat transfer in such devices at the micro-channel scale is very difficult, even impossible. However, the repeatability or even periodicity of the structure suggests the possibility of a macro-scale description following a porous medium approach. Which macro-scale model may be used? This largely remains an open field while some answers have been proposed based on experimental or theoretical work
Investigation of suitability of the method of volume averaging for the study of heat transfer in superconducting accelerator magnet cooled by superfluid helium.
In the field of applied superconductivity, there is a growing need to better understand heat transfers in superconducting accelerator magnets. Depending on the engineering point of view looked at, either 0-D, 1-D, 2D or 3D modeling may be needed. Because of the size of these magnets, alone or coupled together, it is yet, impossible to study this numerically for computational reasons alone without simplification in the description of the geometry and the physics. The main idea of this study is to consider the interior of a superconducting accelerator magnet as a porous medium and to apply methods used in the field of por-ous media physics to obtain the equations that model heat transfers of a superconducting accelerator magnet in different configurations (steady-state, beam losses, quench, etc.) with minimal compromises to the physics and geometry. Since the interior of a superconducting magnet is made of coils, collars and yoke filled with liquid helium, creating channels that interconnect the helium inside the magnet, an upscaling method provides models that describe heat transfer at the magnet scale and are suitable for numerical studies. This paper presents concisely the method and an example of application for super-conducting accelerator magnet cooled by superfluid helium in the steady-state regime in considering the thermal point of view
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